Lithium secondary batteries are widely used in notebook-type personal computers, mobile phones and the like, but they are required to improve lifetime characteristics at high temperature and suppress gas generation inside the battery during high temperature operation.
Lithium secondary batteries are widely used for various applications, and therefore, they need to maintain lifetime characteristics and to suppress gas generation inside the battery even when a usable temperature range is set high. In addition, batteries operating at higher voltage than before are developed and need to maintain life characteristics even at high voltage.
During higher voltage operation than before, a decomposition reaction of the electrolytic solution tends to proceed at the contact portion between the positive electrode and the electrolyte solution. In particular, the decomposition reaction generates gas at high temperature. The generated gas raises the internal pressure of a cell and expands a cell, which leads a problem in practical use. For this reason, it is desired to develop an electrolyte solution with less gas generation, high withstand voltage and high high-temperature durability. Fluorinated solvents and the like are considered as the electrolyte solutions with less gas generation and high withstand voltage. Candidates for the fluorinated solvent include fluorinated carbonates, fluorinated carboxylic acid esters, fluorine-containing ether compounds, fluorine-containing phosphate ester compounds, and the like. The fluorinated solvents, which have low compatibility with other electrolyte solvents and high viscosity, cannot bring about the effects of improving lifetime characteristics and reducing gas generation without optimizing the composition of an electrolyte solution. From this viewpoint, the selection of the composition of an electrolyte solution is important for improving battery characteristics. In addition, it is also necessary to develop electrolyte additives and supporting salts for the electrolyte solution operating at high voltage. As another candidate for the electrolyte solvent, cyclic acetals are available. Although non-fluorinated cyclic acetals are low in oxidation resistance, fluorine-containing cyclic acetals can be the candidates for a solvent for a secondary battery. Patent Document 1 discloses examples using a fluorine-containing cyclic acetal. However, the mixed solvent exemplified in Patent Document 1 cannot bring about sufficient life characteristics. Patent Documents 2 and 3 disclose mixed solvents of a fluorine-containing cyclic acetal and a fluorinated ether, but improvement is required in terms of life characteristics.